Deep desulfurization of transportation fuels is receiving greater attention due to increasingly stringent regulations and fuel specifications for environmental protection purposes. Recently, the U.S. Environmental Protection Agency issued regulations that require refineries to reduce the sulfur content of gasoline from a current average of 300 parts per million by weight (ppmw) to 30 ppmw by 2006, and the sulfur content of highway diesel fuel from a current limit of 500 ppmw to 15 ppmw by 2006.
Deep desulfurization of hydrocarbon fuels to produce ultra-low-sulfur fuel also is motivated by emission-control technologies which are sensitive to sulfur, as well as the need for ultra-low-sulfur fuel for use in fuel cells. Because sulfur is a strong poison to reforming as well as fuel cell catalysts, the sulfur content in liquid hydrocarbon fuels needs to be reduced to an ultra low level, preferably to less than about 10 ppmw for solid oxide fuel cells and to less than about 1 ppmw for polymer electrolyte membrane fuel cells.
Liquid hydrocarbon fuels usually contain sulfur compounds as well as aromatic hydrocarbons at concentrations of about 5-30 wt %. It is well known that naphtha from FCC accounts for over 90% of the sulfur and olefins in gasoline. Sulfur can be removed from FCC by the catalytic hydrodesulfurization (HDS) process. This process, however, requires high consumption of hydrogen and significantly reduces fuel octane number due to olefin saturation. Because gasoline contains olefins which have high-octane value, selective removal of sulfur without loss of olefins is highly desirable.
Although it may be possible to reduce the sulfur content in gasoline to below 30 ppmw by the HDS process, the HDS process is very inconvenient for production of ultra low sulfur content gasoline, particularly for fuel cell applications. This is due in part to the need to use severe operating conditions, including high hydrogen consumption and consequent octane loss. The HDS process also is not suitable for reducing sulfur content in diesel fuel to below 15 ppmw because the remaining sulfur compounds such as 4,6-dimethyldibenzothiophene (4,6-DMDBT) and trimethyl dibenzothiophene (TMDBT) are refractory and very difficult to remove.
A need therefore exists for new methods and materials for deep desulfurization of liquid hydrocarbon fuels to meet environmental concerns as well as to produce ultra-low sulfur fuels for fuel cell applications.